The predominant number of temperature sensors used in industry and research are secondary thermometers. This means that the corresponding sensors, such as e.g. resistance thermometers or thermocouples must be repeatedly calibrated at least prior to their first use and usually also in the course of their regular use. For this purpose the temperature-sensitive sensors or temperature-stabilized switches to be calibrated are compared in furnaces or baths with a standard thermometer. Devices that temper a corresponding calibration volume to a predetermined constant target temperature are known. These so-called temperature calibrators can be designed as heavy immobile devices or, as they are described in the document U.S. Pat. No. 3,939,687 A, as compact portable calibrators.
In order to ensure optimum thermal coupling of the samples to the calibration or test volume, various insert sleeves or sockets that are adapted to the sensors to be tested can be introduced as solid bodies into the calibration volume of the temperature calibrator. For the calibration of sensors with complicated geometries, the calibration volume can be filled with liquid, gaseous or granular calibration media. In order to achieve spatial temperature distribution as constant as possible within the calibration volume, the calibration medium should have the highest possible thermal conductivity. To guarantee a very constant temperature curve, i.e., a high temperature stability, the calibration medium should have the highest possible thermal capacity. Since the calibration volume is to be tempered to the target temperature set by the user, heat can be removed from or added to the calibration volume through a thermally conductive body surrounding the volume. This body is typically designed in immobile calibrators as a tank and in portable calibrators typically as a metallic block and is in thermal contact with heat sinks, such as Peltier elements operated as cooling elements as described in DE 2005 006 710 U1, or the colder ambient air and heat sources, such as a resistance heating or warmer ambient air.
This leads to the question with which intensity or power the adjustable cooling and heating elements (control and manipulated variables) must be operated so that the temperature of the calibration volume (process variable) reaches the desired temperature value (setpoint) as quickly as possible and also holds it as stable as possible even with temporal changes such as the ambient air (disturbance parameter). The regulation technology problem of setting the control variables as a function of the temperatures measured in the calibration volume or in the heat conduction member (measurands) is solved by the present invention.
A well-known approach for controlling heating and cooling systems is the use of one or more associated PID controllers, as described in DE 2023130 B. A general disadvantage of the use of PID controllers is that, at least to achieve optimal control performance, i.e. a high stability of the temperature of the calibration volume, a very fine adjustment or complicated determination of the control parameters is necessary.
Another drawback in the case of the control of a temperature calibrator is that the optimal control parameters are dependent on environmental conditions, such as the ambient temperature, humidity or power supply. However, the main difficulty in the control of temperature calibrators is the large inertia of the controlled system, which extends from the heat sources and heat sinks over the heat conduction part to the calibration volume. Thus, even with relatively slow variations due to the high heat capacity of the heat conduction part, which may be designed as a metal block, and the calibration volume, which can be designed as a metallic insert bushing, the heating power with a frequency of less than 0.1 Hz, a phase lag of the temperature of the calibration volume for the heating capacity of nearly 3π can be observed. Accordingly, a stable control that responds to changes in environmental conditions within about seconds is not possible by means of one or several PID controllers or only after an extensive determination of appropriate control parameters. This has the consequence that the achievable temperature stability for target temperatures above 500° C. with the temperature calibrators available on the market at the time is about ±30 mK, and thus almost an order of magnitude worse than that necessary for high-precision temperature calibrations stability of ±5 mK.
One way to achieve both a high level of temperature homogeneity and a temporal temperature stability is the integration of one or more fixed-point cells in the block of a temperature calibrator as described in the document WO 2013/113683 A2. A disadvantage of the solution is that the constancy of the temperature of the fixed-point cell over the period of the phase transformation is given only for a phase transition temperature of the fixed-point cell used. At the same time the fixed-point cells are expensive so that a device for calibration at different temperature points stabilized by corresponding fixed-point cells would be associated with very high costs.
The dynamic calibration procedure described in document KR 100991021 B1 operates without costly temperature control. Instead, the temperature in the calibration volume near the calibration temperature to be observed is intentionally lowered or increased, and the resulting temperature offset between the normal thermometer and the samples is compensated by a time offset calculation. A disadvantage of this calibration is that the additional uncertainty resulting from the compensation is of the order of ±20 to ±40 mK and thus is significantly larger than the desired ±5 mK. A further disadvantage of the dynamic calibration is that the calibration is not performed at a temperature point, but rather within a temperature interval whose extent cannot be neglected and its location relative to the calibration point under consideration is not defined.